The present invention relates to implantable medical devices and, more particularly, to RF telemetry antennas used in such devices.
As technology has advanced in the development of implantable medical devices, communication with such devices has become increasingly more important. Communication is necessary to program a device, to monitor its various functions and to provide data concerning a patient's response to the device's therapy.
Radio frequency transmissions (RF) are commonly employed to communicate with an implantable medical device. A conventional RF telemetry transmission system generates low amplitude magnetic fields by oscillating current in an LC circuit. An RF telemetry antenna can transmit these signals and a receiving RF telemetry antenna can capture these same signals as induced currents within the receiving antenna.
Typically, an implantable medical device has an antenna which can receive and transmit RF transmission signals. Likewise, a separate external device incorporates an antenna which can receive and transmit RF transmission signals from and to the medical device. The external device may be a “programmer” which can transmit (down-link) and receive (up-link) analog and digital data to and from an implantable medical device. As such, the term “programmer” will be used hereinafter to refer to any external device which can provide and control down-link and/or up-link telemetry transmissions with the implantable medical device. The up-linked data is provided from a register or memory within the implantable device. Similarly, down-linked data can be stored in a register or memory in the implantable device.
Analog data which can be up-linked include values for battery voltage, sampled intracardiac electrocardiogram amplitudes, sensor output signals, pacing pulse amplitudes, energy consumption, pulse-width and pacing lead impedance. Digital data which can be transmitted include statistics related to performance, event markers, current values of programmable parameters, implant data, and patient and device identifier codes. These analog and digital data may be telemetered from many kinds of implantable medical devices, including implantable cardiac pacemakers, cardioverter-defibrillators, drug infusion pumps, cardiac and other physiologic monitors, nerve and muscle stimulators, deep brain stimulators, cochlear implants and artificial hearts.
As implantable medical devices have become more complex and capable of greater data processing, greater performance demands have been placed on the device's telemetry transmission system. It is desirable to have a telemetry system that permits large quantities of data to be transmitted in the shortest time interval, while maintaining high transfer reliability. Greater processing capability also, unfortunately, generally translates to higher usage of available power. Thus, there are competing factors which must be considered in designing an RF telemetry system. Such a telemetry system should ideally provide reliable and rapid transmission, yet conserve the device's battery power.
An important component of the RF telemetry system is the transmitting antenna. Use of RF antennas are common in the non-medical arts as provided in the following U.S. Pat. Nos. 5,912,648; 6,008,762; 6,054,955; 6,133,890; 6,218,992 B1; 6,259,418 B1; 6,285,336 B1; 6,317,099 B1; and 6,342,857 B1. Use of an RF telemetry antenna in an implantable medical device presents unique challenges because such a device is implanted in body tissue and the antenna must fit on or inside the compact housing of the medical device.
In addition, there are imposed regulatory limitations on permissible transmission frequencies and radiating power. Medical devices have been allocated a set frequency band in the UHF spectrum, specifically from 402 to 405 MHz, by the United States FCC (Federal Communications Commission), by the CEPT (Conference Européenne des Administration des Postes et des Telecommunications, i.e., the European Conference of Postal and Telecommunications Administrations) and by MICS (Medical Implant Communications Service). These agencies have also set a standard for maximum allowable Equivalent Isotropically Radiated Power (EIRP) emanating from an implantable medical device at a very low 25 μW. It is necessary, therefore, to have an efficient antenna for receiving this weak signal. At the same time, the antenna should have a relatively wide bandwidth of at least 3 MHz, so as to efficiently capture a telemetry signal anywhere within the allowable 402 to 405 MHz frequency band.
One factor affecting performance of the antenna is the housing of an implantable medical device, which housing is typically a conductive metal such as titanium. This metal housing affects the choice of antenna topology. The presence of a metal housing can adversely attenuate the radiated RF field and severely limit the operable distance for effective data transfer between the programmer and the implanted medical device to as short as a few inches.
Another factor that affects the performance of the antenna is the fact that the medical device is placed in surrounding body tissues which can change the RF characteristics of the antenna. Medical devices can be implanted in various parts of the body. For example, cardiac pacemakers can be implanted close to the clavicle and near the surface of the skin. Other devices, such as spinal cord stimulators, may be implanted in the abdomen under a thick layer of muscle, fascia and skin. These interposing body tissues can change the effective wavelength of the RF signals received by an antenna in the implanted device. Body tissues are conductive dielectric materials, i.e., they have relatively high dielectric constants (ξr) and exhibit conductivity (σ) which results in significant signal losses. For example, the conductivity of muscle tissue is between about 0.05 and 1.4 S/m. The dielectric constant can provide a measure of the reduction of the wavelength within a tissue. The dielectric constant of muscle tissue (in the UHF band) is between 50 and 80, while for comparison, the dielectric constant of air is about 1. Since the wavelength is reduced in body tissue, the electrical length of the antenna is effectively increased. The dielectric constants and conductivities of different tissues also vary with the frequency of the RF transmission signal. Increasing the frequency of the transmission signal increases the signal propagation loss. The losses owing to tissue conductivity are balanced to some extent by an efficiency gained from the wavelength reduction in the tissue.
If the antenna design (and the telemetry system) can only operate in a very narrow bandwidth, data transmission may not be reliable in clinical use. A broader bandwidth enables information to be transmitted more quickly and with greater reliability. In addition the assigned bandwidth between 402 and 405 MHz is divided into 10 channels in 300 KHz increments. Transmitting over many channels using one antenna provides greater flexibility for the telemetry system. To take full advantage of all channels available, it is desirable to have an antenna which is able to receive and send transmissions over the full frequency range of 402 to 405 MHz.
Still another factor which affects the antenna performance is the size of an implantable, medical device which the antenna must be attached to or placed in. Container size is problematic, for example, with a standard monopole antenna which is likely to be too large. A monopole antenna has a conductor material which has a free end and an end which is electrically connected. This antenna should have a length which is one-quarter of a wavelength (λ) long and, as is, this antenna is not practical for use in an implantable medical device. Specifically, a monopole that is shorter than λ/4 will not resonate at the desired frequency of around 403 MHZ and will require extra circuit components to tune the antenna to achieve the desired resonance frequency. The use of extra tuning components will result in loss of needed battery power.
A number of other antenna designs have been made or proposed to solve the design constraints of implantable antennas. A conventional example is a loop antenna which is used in commercially available medical devices. A loop antenna has two ends, each end having an electrical connection. While a loop antenna is advantageously compact and can be designed to work in a medical device, there are drawbacks inherent to this design. For example, the design suffers from a relatively low efficiency, a small operating range and a narrow bandwidth. As a result, extra power-consuming circuitry may be necessary to compensate for these deficiencies.
In one design, disclosed in U.S. Pat. No. 4,681,111 to Silvian, a stub antenna associated with the header is employed as the implantable antenna for high carrier frequencies of up to 200 MHz and employing phase shift keying (PSK) modulation. In another design, disclosed in U.S. Pat. Nos. 5,058,581 and 5,562,713 issued to Silvian, an RF telemetry antenna uses the elongated wire conductor of one or more medical leads extending away from the implanted medical device. In the examples provided, the medical lead is a cardiac lead used to deliver electrical stimulation energy to the heart from an implantable pulse generator (IPG) and to conduct electrical heart signals back to a sense amplifier within the IPG. The conductor wire of the medical lead can operate as a far field radiator to a more remotely located programmer RF telemetry antenna. Advantageously, it is not necessary to maintain a close spacing between the programmer RF telemetry antenna and the implanted cardiac lead antenna. Consequently, the antenna is less sensitive to patient movement during telemetry transmission.
There are, however, disadvantages with this lead design. Because the radiating field is maintained by the current flowing in the lead conductor, RF telemetry transmission via the lead conductor may conflict with sensing and stimulation operations. The elongated lead wire, which is the RF telemetry antenna, has directional radiation nulls dependent on the placement direction of the medical lead, which direction can vary with each patient. Both of these factors often necessitate that the up-link telemetry transmission energy be set very high to ensure that the RF transmission is detected at the programmer telemetry antenna. In addition, the design cannot be used in all implantable medical devices, such as drug infusion pumps and artificial hearts, because these do not have stimulating leads extending from the medical device.
A microstrip antenna design is disclosed in U.S. Pat. No. 5,861,019. The antenna is formed on or within the exterior surface of an implantable device housing. The microstrip patch is formed of an electrically conductive radiator patch that is laminated upon one side of a dielectric substrate layer.
Thus, it is apparent that there is a continuing need for a more efficient and compact antenna system that can be used with an implantable medical device having a metal housing.
It is further apparent that there is a need for such an antenna system which has high transmission reliability, conserves battery power, and can operate under other design and regulatory limitations imposed on a body implantable medical device.